The present invention relates to mufflers. The invention particularly concerns methods and arrangements for mufflers which, in addition to normal attenuation duties, are responsible for muffling the types of noise associated with engine retarders, especially engine retarders of the type sometimes referred to as engine compression brake-type systems.
Diesel engine retarders, of the type sometimes called engine compression brakes, are used to slow down vehicles such as trucks, either without the application of the truck""s normal wheel brakes or to enhance braking when used in cooperation with wheel brakes. In trucks which have such engine retarders, operation is generally as follows. First, fuel flow to the engine is shut off so as to stop the combustion process and subsequent power generation. Next, a device in the engine valve train opens the exhaust valve a slight amount at the end (top) of the usual compression stroke. As a result, the engine is turned into a very inefficient pump. The energy input to this pump, i.e. to the engine, comes from the inertia of the moving truck through the power train (transmission, axles, wheels, etc.). This pumping process (pump work) significantly slows the moving truck.
A typical compression-type brake can be understood by comparing it with a four-cycle engine that does not have a compression-type brake system. (It is noted, however, that most compression brake-type systems are useful on both two and four-cycle diesel engines.) Without a compression-type brake, on stroke 1, called the induction stroke, the piston moves down and an inlet valve opens. This draws air into the cylinder. If there is a turbo charger, the air is forced into the cylinder by boost pressure from the turbo charger. On stroke 2, called the compression stroke, the inlet valve closes and the piston moves up. The fuel mixture is thus compressed. The energy required to compress this air is produced by the driving wheels of the vehicle. On stroke 3, called the power stroke, fuel is injected into the cylinder, in turn igniting due to compression, forcing the piston back down the cylinder. As the piston is forced back down the cylinder, the energy is returned to the driving wheels. On stroke 4, called the exhaust stroke, the exhaust valve opens and the piston rises, pushing the exhaust gases out of the cylinder.
With a compression-type brake system, the typical four-cycle engine is modified from that described above. With a compression-type brake activated, on the compression stroke the inlet valve opens, and air is drawn or forced into the cylinder from the intake manifold. This is no different from the typical induction stroke. On the compression stroke, air is compressed to approximately 500 psi or higher by the engine piston. The energy required to compress the air is produced by the inertia of the truck""s driving wheels. During the compression stroke, near top dead center, the compression-type brake opens the exhaust valves, venting the high pressure air and dissipating the stored energy through the exhaust system. In the power stroke, essentially no energy is returned to the piston, and thus, essentially no energy is returned to the driving wheels. There is a loss of energy. This loss is the engine retarding work done. During the exhaust stroke, the outlet valve opens and the piston rises, pushing the exhaust gases out of the cylinder. The exhaust stroke, during operation of a compression-type brake is no different than the exhaust stroke of a normal diesel engine.
Typically, trucks with engine retarders are provided with an overall on/off control switch in the truck cab. That is, the engine retarder is left xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d by the driver; and, when the retarder is xe2x80x9conxe2x80x9d it will automatically engage when the driver takes pressure off the accelerator pedal or when pressure is applied to the wheel brakes, depending upon the system. Application of a compression brake-type engine retarder can produce as much or more power to stop the vehicle, than the engine can produce during normal operation. This is considered beneficial by truck operators in many instances, since it significantly reduces brake wear while still serving as an effective brake.
A major manufacturer of such engine retarders in the United States is Jacobs Vehicle Systems of Bloomfield, Conn. The systems manufactured by, or under the direction of, Jacobs Vehicle Systems, are generally available under the trademark xe2x80x9cJake Brakexe2x80x9d. At the present time, Jake Brake(copyright) Systems, or similar engine retarders, are found on many trucks, either installed by the manufacturer (for example, Freightliner, Peterbilt, Mack), or installed afterwards, by choice of the truck owner.
The use of such compression brake engine retarders, although considered highly effective for braking and safety, is associated with undesirable noise. In particular, compression brake operation is associated with a very distinctive, high amplitude, staccato noise or engine xe2x80x9cbarkxe2x80x9d. This noise is of a nature that cannot be adequately muffled, by conventional truck muffler systems. The noise is often so objectionable that in many municipalities, especially in hilly areas, signs are posted prohibiting the use of compression brake-type engine retarders.
In certain applications, this disclosure is directed to muffler arrangements effective for muffling engine compression brake-type systems. Certain muffler arrangements, in accordance with this aspect of the disclosure, include an outer wall, usually cylindrical, defining an internal volume, and an inlet and outlet tube oriented within the internal volume of the outer wall. In typical arrangements, the outlet tube defines a sonic choke. An inner, perforated wall is spaced from the outer wall, to define an annular volume therebetween. The annular volume may include a packing, or padding, of absorptive material within the annular volume. The packing material within the annular volume provides an absorptive function, and helps reduce drumming of the outer wall or shell.
In certain arrangements, the inner perforated wall and annular volume is in alignment with the inlet region of the muffler. That is, the first, inner perforated wall may circumscribe at least a portion of the inlet tube.
In one preferred arrangement, at least one second volume of packing material is positioned against and around a section of the outlet tube construction. Preferably, the second volume of packing material is positioned spaced from the outer wall or shell.
In one embodiment, a third volume of packing material is positioned against and around a section of the inlet tube. Preferably, the third volume of packing material is positioned spaced from the outer wall or shell. Preferably, the first volume of packing material in the first annular volume circumscribes both the inlet tube construction and outlet tube construction, with the packing materials positioned thereagainst. Other embodiments include more volumes of packing material positioned against the outlet tube.
Muffler constructions in accordance with the principles characterized herein have been found to perform desirable muffling functions at high frequency octave band values; that is, octave bands in a frequency range in which prior art muffler constructions have not adequately muffled. Certain applications described herein include trucks with high horsepower engines and equipped with engine compression brake-type engine retarders and exhaust mufflers which muffle objectionable noises emitted from the truck during operation of the compression brake-type engine retarder.
In certain applications, this disclosure is directed to a method for muffling exhaust noise from a truck during operation of a compression-type brake using a muffler. The truck typically has an engine rated for operation, typically at some rpm between 1,800 rpm and 2,100 rpm, inclusive, for a power of at least 500 hp. The preferred muffler is cylindrical with an outside diameter of no greater than about 11 inches and an overall length of no greater than 60 inches. The method includes a step of muffling noise, during operation of the compression brake-type engine retarder to an overall sound pressure level of no greater than 68 dba. Muffler constructions of the type described herein may be used to accomplish this method.
Muffler arrangements are described that are effective for muffling engine compression brake-type systems. Certain muffler arrangements described herein achieve enhanced performance at low frequencies, such as 125 Hz and 63 Hz.
In one arrangement, there is an outer shell wall, an inner perforated wall, a region of packing material positioned between the perforated wall and the outer wall, a second inner wall spaced from a perforated section of an outlet tube, and a second region of packing material positioned between the second inner wall and the perforated section of the outlet tube.
Another muffler construction includes a first region of packing material positioned between an outermost wall and an inner perforated wall, and a second region of packing material positioned around a perforated section of a tubular extension of an outflow tube. The outflow tube may include both the tubular extension and an outlet tube section, wherein the outlet tube section circumscribes the tubular extension.
In certain preferred arrangements, the outlet tube includes a perforated section that is spaced from an internal end of the outlet tube a distance of at least 20 percent of a total axial length of the outlet tube construction. In certain preferred embodiments, this first perforated section is spaced a distance from the internal outlet tube a distance of no greater than 50 percent of a total axial length of the outlet tube construction.